A conventional flash EEPROM memory cell typically includes spaced source and drain regions diffused into a semiconductor substrate and a channel region provided therebetween. In addition, the conventional flash memory cell includes an electrically isolated floating gate provided over the channel region, and a control gate disposed above the floating gate. By applying appropriate voltages to the source, drain and control gate, charge is either stored on or removed from the floating gate, and thus data, in the form of such charge, is stored in, or erased from, the memory cell. The presence or absence of charge on the floating gate determines whether current flows between the source and drain regions when the memory cell is selected. Such current can be sensed by appropriate circuitry as a binary “1” stored in the memory cell. Alternatively, if no current is sensed, a binary “0” may assumed to be stored in the memory cell.
When the memory cell is deselected, however, little or no current should flow in the channel region of the memory cell.
In order to improve memory cell density, flash memory cells have been aggressively scaled downward. As a result, the channel length, i.e., the distance between the diffused source and drain regions, has been substantially reduced. Memory cells having a reduced size, however, can suffer from so-called “short channel effects.” In particular, as the channel region decreases in size, pn junction depletion regions formed about the source and drain regions can create relatively strong electric fields such that the current flowing through the cell is not controlled by biases applied to the control gate and the presence/absence of charge on the floating gate. Rather, the current is determined primarily by the voltage across the source and drain regions. As a result, for a given source and drain voltage, an excessively high current may flow in a deselected memory cell resulting in memory read out errors.
The present invention is directed to overcome one or more of the problems of the prior art.